Harley and Ivy: Roomies! Context: Following a mass escape from Arkham triggered by a Ra's al Ghul plot (kill of his Rogues Gallery so that Bruce would have enough time to date Talia), the various Bat villains are being sent to a regular prison while Arkham is being repaired.
During the mass escape from Arkham, Ivy was revealled to be a plant-like creature and not technically a human being any more. Harley is upset that Ivy wouldn't share this with her as she was meant to be her best friend, so she leaves her trapped under some rubble as she legs it.
Here is when the two meet again after Harley is recaptured.